Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016
Statement of Financial Position [Abstract]
Interest-earning deposits	$ 128,460	$ 267,829
Available-for-sale securities, amortized cost	492,395	517,791
Held-to-maturity securities, estimated fair value	1,027,292	1,122,867
Loans receivable, allowance for credit losses	$ 8,521	$ 8,540
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	100,000,000	100,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	1,400,000,000	1,400,000,000
Common stock, shares issued	137,915,672	137,486,172
Common stock, shares outstanding	137,915,672	137,486,172